1. Field of the Invention
The present invention relates to a coating material for the inner surface of a cathode-ray tube. More particularly, the invention relates to a coating material used for forming an electroconductive coating, which scarcely releases gases during an exhausting-baking step and has a good gas-adsorbing property.
2. Description of Prior Art
Cathode-ray tubes are generally manufactured according to the following method. In the first place, a funnel section and a panel section are put together with a bonding agent of frit glass that is set between them. The inside of the funnel section is previously applied with an electroconductive coating (herein-after referred to as xe2x80x9cinner coatingxe2x80x9d) and the panel section is previously provided with a fluorescent screen. Then, they are baked at about 450xc2x0 C. to unify the funnel section and the panel section into a tubular body. Subsequently, an electron gun is built into the tubular body and the inside of the tube is evacuated through a tip tube that connects its neck section with a vacuum pump, while heating up to about 400xc2x0 C. to exhaust needless gases from inside the tube. This step is called exhausting-baking step. After that, the tip tube is sealed up and cut to make the tubular body a closed system, then a substance of getter such as barium is scattered inside the tubular body to increase the degree of vacuum, thereby completing a cathode-ray tube.
The service life of the cathode-ray tube made according to the above method is closely related to the degree of vacuum inside the tube, and the degree of vacuum depends on the nature of inner coating. In other words, when the degree of vacuum in a cathode-ray tube is so low that there is a large quantity of needless gases in the tube, the ability of the cathode to release electrons becomes weak, and ultimately, the emission of electrons from the cathode is damaged. This is due to the fact that the electron beam emitted during the operation of cathode-ray tube ionizes needless gases to exert a harmful influence on the cathode.
Concerning the relationship between the degree of vacuum and the inner coating, when a large quantity of gas is released from the inner coating in an exhausting-baking step, the exhaustion cannot be completed during the exhausting-baking step. As a result, released gases remain in the tube to cause the reduction of the degree of vacuum. However, because the inner coating has the ability as getter, it adsorbs the needless gas in the tube to increase the degree of vacuum.
The properties of inner coating of this kind depend on the composition of coating material on the inner surface of a cathode-ray tube. In general practice, the inner coating is formed by applying a coating material to the inner surface of a funnel by means of spraying, brushing, flow coating or the like, and it is then dried. The coating material for inner surface used here is generally made by suspending or dispersing an electroconductive substance of graphite particles and particles of metal compounds to regulate the electrical resistance, in an aqueous medium that contains a dispersing agent and alkali silicate as an adhesive. The metal compounds are exemplified by metal oxides and metal carbides such as iron oxide, titanium oxide and silicon carbide.
Among the components of the above coating materials, alkali silicate compound is a gas-releasing substance and graphite particles serve as an adsorbent for gases. The reason for that the alkali silicate compound works as a gas-releasing substance is such that the ions of alkali metal of alkali silicate in the inner coating move to the surface of coating according to various conditions, and they combine with carbon dioxide (CO2) and water vapor (H2O) to form hydrogen carbonates or carbonate hydrates. It is supposed that these products are subjected to thermal decomposition by heating in the exhausting-baking step to generate gases such as carbon dioxide and water vapor.
Incidentally, it is well known as a technical art for suppressing the movement of alkali ions derived from alkali silicate of an electroconductive coating, i.e., inner coating, to utilize xe2x80x9cmixed alkali effectxe2x80x9d that is produced when two or more kinds of alkali metals are mixed in a glass. This xe2x80x9cmixed alkali effectxe2x80x9d is disclosed, for example, in the publication by Masayuki Yamane, xe2x80x9cFor people who make glass for the first timexe2x80x9d, issued Jul. 10, 1989, published Uchidarokakuho, p. 85-86.
A foregoing art in applying the xe2x80x9cmixed alkali effectxe2x80x9d to the inner coating of cathode-ray tube related to the present invention is disclosed, for example, in Japanese Laid-Open Patent Publication No. 52-52362 (1977). That is, the quantities of gases adsorbed from the atmosphere such as H2O, CO2, etc. are suppressed by using an inner coating that contains a bonding agent of silicates consisting of sodium silicate and/or potassium silicate and lithium silicate to reduce the quantity of gases released from the coating. However, the invention intends to improve the gas-releasing property, but does not take the adsorption of needless gases into consideration.
In the following, it will be described that graphite particles work as a gas adsorbent. The reason for this effect has not been made clear, but according to the report by Hashiba, et al. (J. Vac. Soc. Jpn., 42 [12] (1999) p. 70-75), graphite has actually an adsorbing effect. In addition, an example of applying the adsorbing effect of graphite to the production of fluorescent display tube is described in Japanese Laid-Open Patent Publication No. 57-136747 (1982).
As mentioned above, in alkali silicate compounds as gas-releasing source in particular, various properties such as viscosity and film-forming property change largely depending on the kinds and the compositions of alkali metals composing the salts and on the ratio of silicon dioxide to alkali metal oxides. Therefore, with a simple technique as used in the above conventional method, it is impossible to make the best use of the ability of graphite of adsorbing and to attain a sufficient effect.
In view of the above technical background, both the gas-releasing property and the gas-adsorbing property of the inner coating have been fully taken into consideration. As a result, the object of the present invention is to provide a coating material for the inner surface of a cathode-ray tube that is most suitable for increasing the degree of vacuum in the tube by suppressing the quantity of gas released from the inner coating and making the best use of the adsorbing ability of graphite.
This invention, thus, relates to a coating material for the inner surface of a cathode ray tube which comprises an aqueous dispersion medium containing alkali silicate compounds consisting of lithium and potassium and a dispersing agent, and graphite particles or a composition of graphite particles and metal oxide particles or metal carbide particles suspended therein, wherein the molar ratio of potassium to lithium (K/Li) in the dispersion medium is in the range of 1 to 9, and the molar ratio (SiO2/R2O) of silicon dioxide (SiO2) in the dispersion medium to the total alkali metal oxides (R2O) that is converted taking the contents of lithium and potassium in the dispersion medium, is in the range of 2.5 to 3.5.